I call him Sir, but only on what you might call special occasions: when we play/during a scene, for the daily caning or whatever other torment/discipline, in the heat of the moment (oh god, oh sir...), when i ask permission for anything, if begging seems to be called for, or whenever he gets that look and Sir seems like a wise choice. Sometimes, in the middle of the day-to-day, he changes the mood of things by suddenly insisting on it - it's a signal and it effects the shift in the moment.
Sir isn't ever used in front of other people or within earshot even. And i use it infrequently enough that it isn't my first impulse when speaking to or about him. I wouldn't really ever slip up and call him Sir out of context because it's more of a thing i have to think about to remember rather than the automatic thing.
We aren't formal or protocol - not even very, very low protocol. We are entirely natural and i guess equal in our communications. I'm not quite sure how to describe it: our dynamic isn't expressed at all in our everyday interactions, not in our speech, or manners, or gestures, not overtly, at all. There would be, to any observer, no sense of me being 'in my place.'
Tori included her perspective on this in an excellent post. I can easily see how using Sir, or some other title of respect or deference, could have a profound impact on my frame of mind, the way i communicate with him, and how i think of him all around. We are most often completely at ease with each other, like i said - no overt expression of us. But, there are times that i feel way too at ease, way too familiar maybe, not quite disrespectful, but pushing the boundary.... if that makes any sense, considering that there are no expectations like that in the first place. It's entirely situational, and i know exactly when i'm doing it, and i think he does too.
So i can see that a requirement about how i address him, or a protocol around speech, or manners or gestures or any such thing, would serve to reinforce our dynamic - which i like and often need. It would reassure me when i doubt it, and remind me when i choose to ignore it. I have learned the hard way never to say never - but i don't think this is an area where he will choose to make changes. I do know it would be very difficult for me.
He called the other day while I was in the middle of a conversation with my son. I told him i was talking with the son so he (my husband) and i wrapped up quickly. At the end he insisted i say "yes sir." I reminded him the son was standing right there, but he insisted. I giggled a little and tried to weasel around it - no go - so i said it. My son's eyebrows shot up and then he rolled his eyes. Those two words were painfully difficult and stopped me in my tracks. I can't imagine Sir or anything like that just rolling off my tongue in front of other people ever.